High Affinity Sigma Research Articles

Sigma receptor 1 activation attenuates release of inflammatory cytokines MIP1γ, MIP2, MIP3α, and IL12 (p40/p70) by retinal Müller glial cells.

The high-affinity sigma receptor 1 (σR1) ligand (+)-pentazocine ((+)-PTZ) affords profound retinal neuroprotection in vitro and in vivo by a yet-unknown mechanism. A common feature of retinal disease is Müller cell reactive gliosis, which includes cytokine release. Here, we investigated whether lipopolysaccharide (LPS) stimulates cytokine release by primary mouse Müller cells and whether (+)-PTZ alters release. Using a highly sensitive inflammatory antibody array we observed significant release of macrophage inflammatory proteins (MIP1γ, MIP2, MIP3α) and interleukin-12 (IL12 (p40/p70)) in LPS-treated cells compared to controls, and a significant decrease in secretion upon (+)-PTZ treatment. Müller cells from σR1 knockout mice demonstrated increased MIP1γ, MIP2, MIP3α and IL12 (p40/p70) secretion when exposed to LPS compared to LPS-stimulated WT cells. We investigated whether cytokine secretion was accompanied by cytosolic-to-nuclear NFκB translocation and whether endothelial cell adhesion/migration was altered by released cytokines. Cells exposed to LPS demonstrated increased NFκB nuclear location, which was reduced significantly in (+)-PTZ-treated cells. Media conditioned by LPS-stimulated-Müller cells induced leukocyte-endothelial cell adhesion and endothelial cell migration, which was attenuated by (+)-PTZ treatment. The findings suggest that release of certain inflammatory cytokines by Müller cells can be attenuated by σR1 ligands providing insights into the retinal neuroprotective role of this receptor.

Carboline- and phenothiazine-derivated heterocycles as potent SIGMA-1 protein ligands.

Sigma 1 receptors are associated with neurodegenerative and psychiatric disorders. These receptors, via their chaperoning functions that counteract endoplasmic reticulum stress and block neurodegeneration, may serve as a target for a new generation of antidepressants or neuroprotective agents. The involvement of these receptors has also been observed in neuropathic pain and cancer. Only a few ligands, such as Igmesine and Anavex 2-73, have been involved in clinical trials. Thus the development of sigma 1 ligands is of interest to a new generation of drugs. Previous work in our lab underlined the potency of benzannulated bicyclic compounds as interesting ligands. Herein the work was extended to a series of novel tricyclic compounds. Carboline- and phenothiazine-derivated compounds were designed and synthesized. Invitro competition binding assays for sigma 1 and 2 receptors showed that most of them have high affinity for sigma 1 receptor (Ki=2.5-18nM), and selectivity toward sigma 2 receptor, without cytotoxic effects on SY5Y cells.

Opipramol Overdose Presented with Wide-Complex Tachycardia to the Emergency Department

Opipramol is an antidepressant and anxiolytic drug similar in structure to tricyclic antidepressants (TCAs) but it is primarily used for the treatment of generalized anxiety disorders. Unlike many TCAs, opipramol has no reuptake-inhibiting properties. Opipramol acts as a high affinity sigma receptor agonist. Opipramol acts as a low to moderate affinity antagonist for the dopamine-2 (D2), 5-hydroxtryptamine (5-HT2), histamine 1 (H1), histamine 2 (H2), and muscarinic acetylcholine receptors accounting for its antihistamine effects, and muscarinic anticholinergic properties. Here, we report a rare case of opipramol overdose case involving a 18-year-old woman, presenting to our emergency department (ED) with loss of consciousness and wide-complex tachycardia. A firm diagnosis of opipramol overdose was made on the basis of clinical, laboratory and electrocardiogram findings.

Neuroprotective effects of high affinity sigma 1 receptor selective compounds

We previously reported that the antipsychotic drug haloperidol, a multifunctional D2-like dopamine and sigma receptor subtype antagonist, has neuroprotective properties. In this study we further examined the association between neuroprotection and receptor antagonism by evaluating a panel of novel compounds with varying affinity at sigma and D2-like dopamine receptors. These compounds were evaluated using an in vitro cytotoxicity assay that utilizes a hippocampal-derived cell line, HT-22, in the presence or absence of varying concentrations (5 to 20mM) of glutamate. While haloperidol was found to be a potent neuroprotective agent in this in vitro cell assay, the prototypic sigma 1 receptor agonist (+)-pentazocine was found not to be neuroprotective. Subsequently, the potency for the neuroprotection of HT-22 cells was evaluated for a) three SV series indoles which have nMolar affinity at D2-like receptors but varying affinity at sigma 1 receptor and b) two benzyl phenylacetamides sigma 1 receptor selective compounds which bind with low affinity at D2-like receptors but have nMolar affinity for the sigma 1 receptor. We observed that cytoprotection correlated with the affinity of the compounds for sigma 1 receptors. Based upon results from the HT-22 cell-based in vitro assay, two phenylacetamides, LS-127 and LS-137, were further evaluated in vivo using a transient middle cerebral artery occlusion (t-MCAO) model of stroke. At a dose of 100μg/kg, both LS-127 and LS-137 attenuated infarct volume by approximately 50%. These studies provide further evidence that sigma 1 receptor selective compounds can provide neuroprotection in cytotoxic situations. These results also demonstrate that sigma 1 receptor selective benzyl phenylacetamides are candidate pharmacotherapeutic agents that could be used to minimize neuronal death after a stroke or head trauma.

CM156, a high affinity sigma ligand, attenuates the stimulant and neurotoxic effects of methamphetamine in mice

Methamphetamine (METH) is a highly addictive psychostimulant drug of abuse. Low and high dose administration of METH leads to locomotor stimulation, and dopaminergic and serotonergic neurotoxicity, respectively. The behavioral stimulant and neurotoxic effects of METH can contribute to addiction and other neuropsychiatric disorders, thus necessitating the identification of potential pharmacotherapeutics against these effects produced by METH. METH binds to σ receptors at physiologically relevant concentrations. Also, σ receptors are present on and can modulate dopaminergic and serotonergic neurons. Therefore, σ receptors provide a viable target for the development of pharmacotherapeutics against the adverse effects of METH. In the present study, CM156, a σ receptor ligand with high affinity and selectivity for σ receptors over 80 other non-σ binding sites, was evaluated against METH-induced stimulant, hyperthermic, and neurotoxic effects. Pretreatment of male, Swiss Webster mice with CM156 dose dependently attenuated the locomotor stimulation, hyperthermia, striatal dopamine and serotonin depletions, and striatal dopamine and serotonin transporter reductions produced by METH, without significant effects of CM156 on its own. These results demonstrate the ability of a highly selective σ ligand to mitigate the effects of METH.

Interaction of the σ2 Receptor Ligand PB28 with the Human Nucleosome: Computational and Experimental Probes of Interaction with the H2A/H2B Dimer

Sigma-2 (sigma(2)) binding sites are an emerging target for anti-neoplastic agents due to the strong apoptotic effect exhibited by sigma(2) agonists in vitro and the overexpression of these sites in tumor cells. Nonetheless, no sigma(2) receptor protein has been identified. Affinity chromatography using the high-affinity sigma(2) ligand PB28 and human SK-N-SH neuroblastoma cells was previously utilized to identify sigma(2) ligand binding proteins, specifically histones H1, H2A, H2B, and H3.3a. To rationalize this finding, homology modeling and automated docking studies were employed to probe intermolecular interactions between PB28 and human nucleosomal proteins. These studies predicted interaction of PB28 with the H2A/H2B dimer at a series of sites previously found to be implicated in chromatin compaction and nucleosomal assembly. To experimentally verify this prediction, a competitive binding assay was performed on the reconstituted H2A/H2B dimer using [(3)H]PB28 as radioligand, and an IC(50) value of 0.50 nM was determined for PB28 binding. In addition, [(3)H]PB28 was found to accumulate with up to a fivefold excess in nuclear fractions over cytosolic fractions of SK-N-SH and MCF7 cells, indicating that PB28 is capable of entering the nucleus to interact with histone proteins. In conjunction with computational results, these data suggest that PB28 may exert its cytotoxic effect through direct interaction with nuclear material.

1-Cyclohexylpiperazine and 3,3-Dimethylpiperidine Derivatives as Sigma-1 (σ1) and Sigma-2 (σ2) Receptor Ligands: A Review

Herein the evolution in the development of new sigma (sigma) receptor ligands since the middle '90s by our research group is reported. In the effort to contribute to the identification of the structural features for high-affinity ligands selective versus serotonin, dopamine and other CNS-related receptors, two general classes of (naphthalene)alkylamine compounds were prepared and explored, with the aim of addressing the affinities toward the two recognized sigma receptor subtypes. The common template of these compounds was mainly an unsubstituted or methoxy-substituted naphthalene or tetralin nucleus, linked by an alkyl spacer to a substituted piperazine or piperidine ring. The design of new ligands was thought keeping in mind their possible application as PET diagnostic tools and fluorescence tools. High-affinity sigma(2) receptor ligands were found among N-cyclohexylpiperazine derivatives, such as 1-cyclohexyl-4-[3-(5-methoxy-1,2,3,4-tetrahydronaphthalen-1-yl)propyl]piperazine (3) (PB 28), when they were assayed in radioligand binding with [(3)H]-DTG in rat liver. Unfortunately, these ligands were all devoid of a significant selectivity relative to sigma(1) receptor whose binding was assayed with (+)-[(3)H]-pentazocine in guinea pig brain. Nevertheless, compound 3 had previously shown to be 40-fold selective with a slightly different binding method in animals' tissues. Moreover, it demonstrated 46-fold and 59-fold sigma(2) versus sigma(1) receptor binding selectivity in MCF7 and MCF7 ADR tumor cell lines respectively. In the class of piperazines, also high-affinity sigma(1) receptor ligands were found, possibly due to the presence of a double N-atom and an additional reverse mode of binding. Piperidine derivatives were investigated as high-affinity and selective sigma(1) receptor ligands leading to some 3,3-dimethylpiperidines such as 3,3-dimethyl-1-[3-(6-methoxynaphthalen-1-yl)propyl]piperidine (69) which resulted to be highly selective relative to the sigma(2) receptor. For the best ligands, functional assays were conducted in order to investigate agonist/antagonist activity. The effect of chirality in the intermediate methyl-alkyl chain was explored for a class of 4-methylpiperidines linked to some (4-chlorophenoxy)alkyl moieties, and compound (-)-(S)-92 emerged as the most selective sigma(1) relative to sigma(2) receptor ligand.

Induction of hypothermic conditions associated with increased micronuclei formation in sigma‐1 receptor knockout mice after administration of the antipsychotic compound E‐5842

The antipsychotic sigma-1 (sigma(1)) receptor ligand E-5842 has been shown to increase micronucleated polychromatic erythrocyte (MNPCE) frequency in mouse bone marrow secondary to compound-induced hypothermia. Interaction with sigma(1) receptor has been considered a plausible contributing factor for E-5842-induced hypothermia, raising concern for a possible class effect of sigma receptor ligands in the mouse micronucleus (MN) test. We assessed the potential of E-5842 (200 mg/kg, oral) to produce hypothermic conditions associated with increased micronuclei formation in sigma(1) receptor knockout (sigma(1)R-KO) and wild type (WT) mice. After administration, animal's rectal temperature was recorded and peripheral blood and bone marrow samples were obtained (48 hr) and assessed for induction of micronucleated reticulocytes (MNRET) and MNPCE, respectively. E-5842 administration produced marked hypothermia both in sigma(1)R-KO and WT mice. Maximum decreases from preadministration temperature were 12.2 and 13.5 degrees C in sigma(1)R-KO and WT mice, respectively. Temperature returned to normal approximately 32 hr after administration. Bone marrow examination revealed a statistical significant increase (P < 0.05) in MNPCE frequency both in sigma(1)R-KO and WT animals. Examination of peripheral blood samples showed a slight, although nonstatistical significant, increase in MNRET frequency in sigma(1)R-KO mice. No similar effect was observed among WT animals. The results obtained after E-5842 administration to sigma(1)R-KO mice indicate that induction of hypothermic conditions associated with increased MNPCE formation is not mediated by compound interaction with sigma(1) receptor, ruling out concern for a possible class effect of similar high affinity sigma(1) receptor ligands in the mouse MN test.

A new sigma ligand, (±)-PPCC, antagonizes kappa opioid receptor-mediated antinociceptive effect

The compound (1 R,2 S/1 S,2 R)-2-[4-hydroxy-4-phenylpiperidin-1-yl)methyl]-1-(4-methylphenyl) cyclopropanecarboxylate [(±)-PPCC] is a ligand with high affinity for sigma (σ) sites of which the selectivity towards several other receptor systems has been demonstrated. Given the existence of a relationship between the σ system and the kappa opioid (KOP)-mediated analgesia, to characterize the pharmacological properties of (±)-PPCC we analyzed its influence on the analgesic effect of the systemic injected kappa agonist (−)-U-50,488H comparing the effects with those shown by (+)-pentazocine and BD1047. The results demonstrate that the systemic administration of (±)-PPCC (1 mg/kg s.c.) does not modify basal tail-flick latency. Pre-treatment with (±)-PPCC, at the same dose, significantly decreased the antinociceptive effect of (−)-U-50,488H, analogously to the σ compounds used. This study confirms that (±)-PPCC plays the role of σ agonist in this model and strengthens the hypothesis of the σ receptor modulatory role on KOP-mediated analgesia.

The anti‐amnesic and neuroprotective effects of donepezil against amyloid β25‐35 peptide‐induced toxicity in mice involve an interaction with the σ1 receptor

The acetylcholinesterase inhibitor, donepezil, is also a high affinity sigma(1) receptor agonist. We examined the involvement of sigma(1) receptors in its anti-amnesic and neuroprotective properties against amyloid beta(25-35) peptide-induced toxicity in mice. Mice were given an intracerebroventricular (i.c.v.) injection of Abeta(25-35) peptide (9 nmol) 7-9 days before being tested for spontaneous alternation and passive avoidance. Hippocampal lipid peroxidation was measured 7 days after Abeta(25-35) injection to evaluate oxidative stress. Donepezil, the sigma(1) agonist PRE-084 or the cholinesterase (ChE) inhibitors tacrine, rivastigmine and galantamine were administered either 20 min before behavioural sessions to check their anti-amnesic effects, or 20 min before Abeta(25-35) injection, or 24 h after Abeta(25-35) injection and then once daily before behavioural sessions, to check their pre- and post-i.c.v. neuroprotective activity, respectively. All the drugs tested were anti-amnesic, but only the effects of PRE-084 and donepezil were prevented by the sigma(1) antagonist BD1047. Only PRE-084 and donepezil showed neuroprotection when administered pre i.c.v.; they blocked lipid peroxidation and learning deficits, effects inhibited by BD1047. Post i.c.v., PRE-084 and donepezil showed complete neuroprotection whereas the other ChE inhibitors showed partial effects. BD1047 blocked these effects of PRE-084, attenuated those of donepezil, but did not affect the partial effects of the other ChE inhibitors. The potent anti-amnesic and neuroprotective effects of donepezil against Abeta(25-35)-induced toxicity involve both its cholinergic and sigma(1) agonistic properties. This dual action may explain its sustained activity compared to other ChE inhibitors.

Sigma Receptor Activation Reduces Infarct Size at 24 Hours After Permanent Middle Cerebral Artery Occlusion in Rats

The only available treatment for embolic stroke is recombinant tissue plasminogen activator, which must be administered within three hours of stroke onset. We examined the effects of 1,3-di-o-tolyguanidine (DTG), a high affinity sigma receptor agonist, as a potential treatment for decreasing infarct area at delayed time points. Rats were subjected to permanent embolic middle cerebral artery occlusion (MCAO) and allowed to recover before receiving subcutaneous injections of 15 mg/kg of DTG at 24, 48, and 72 hours. At 96 hours the rats were euthanized, and brains harvested and sectioned. Infarct areas were quantified at the level of the cortical/striatal and cortical/hippocampal regions in control (MCAO-only) and DTG treated animals using a marker for neurodegeneration, Fluoro-Jade. DTG treatment significantly reduced infarct area in both cortical/striatal and cortical/hippocampal regions by >80%, relative to control rats. These findings were confirmed by immunohistochemical experiments using the neuronal marker, mouse anti-neuronal nuclei monoclonal antibody (NeuN), which showed that application of DTG significantly increased the number of viable neurons in these regions. Furthermore, DTG blocked the inflammatory response evoked by MCAO, as indicated by decreases in the number of reactive astrocytes and activated microglia/macrophages detected by immunostaining for glial fibrillary acidic protein (GFAP) and binding of isolectin IB4, respectively. Thus, our results demonstrate that the sigma receptor-selective agonist, DTG, can enhance neuronal survival when administered 24 hr after an ischemic stroke. In addition, the efficacy of sigma receptors for stroke treatment at delayed time points is likely the result of combined neuroprotective and anti-inflammatory properties of these receptors.

Effects of OPC-14523, a combined sigma and 5-HT1a ligand, on pre- and post-synaptic 5-HT1a receptors

OPC-14523 (OPC) is a novel compound with high affinity for sigma and 5-HT1A receptors that shows 'antidepressant-like' effects in animal models of depression. We have previously demonstrated that OPC produces an increase in 5-HT neurotransmission and a decreased response of 5-HT neurons to the acute administration of paroxetine in the DRN, an effect that appears to be mediated by OPC's 5-HT1A receptor affinity. The current study sets out to investigate more specifically the effects of OPC on 5-HT1A pre- and post-synaptic receptors, to assess whether it acts as an agonist or antagonist. Using an electrophysiological model of in vivo extracellular recordings in anaesthetized rats, the effects of OPC was assessed on pre-synaptic DRN 5-HT1A autoreceptors and post-synaptically on hippocampal 5-HT1A receptors of CA3 pyramidal neurons. OPC applied by microiontophoresis, produced a significant decrease in the firing activity of 5-HT neurons of the DRN and of quisqualate-activated CA3 pyramidal neurons of the dorsal hippocampus. The effects of OPC on 5-HT1A receptors were significantly reduced by the co-application of the 5-HT1A antagonist WAY-100635. In addition, the effects of OPC were not blocked by the injection of the sigma antagonists NE-100 or haloperidol. Therefore, OPC is acting as an agonist on both pre- and post-synaptic 5-HT1A receptors. The current findings combined with previous data on OPC suggest a pharmacological profile that warrants further investigation.

Preclinical acute toxicity studies and rodent-based dosimetry estimates of the novel sigma-1 receptor radiotracer [ 18F]FPS

[ 18F]1-(Fluoropropyl)-4-[(4-cyanophenoxy)methyl]piperidine ([ 18F]FPS) is a novel high affinity (KD = 0.5 nM) sigma receptor radioligand that exhibits saturable and selective in vivo binding to sigma receptors in rats, mice and non-human primates. In order to support an IND application for the characterization of [ 18F]FPS through PET imaging studies in humans, single organ and whole body radiation adsorbed doses associated with [ 18F]FPS injection were estimated from distribution data obtained in rats. In addition, acute toxicity studies were conducted in rats and rabbits and limited toxicity analyses were performed in dogs. Radiation dosimetry estimates obtained using rat biodistribution analysis of [ 18F]FPS suggest that most organs would receive around 0.012-0.015 mGy/MBq. The adrenal glands, brain, kidneys, lungs, and spleen would receive slightly higher doses (0.02-0.03 mGy/MBq). The adrenal glands were identified as the organs receiving the greatest adsorbed radiation dose. The total exposure resulting from a 5 mCi administration of [ 18F]FPS is well below the FDA defined limits for yearly cumulative and per study exposures to research participants. Extended acute toxicity studies in rats and rabbits, and limited acute toxicity studies in beagle dogs suggest at least a 175-fold safety margin in humans at a mass dose limit of 2.8 μg per intravenous injection. This estimate is based on the measured no observable effect doses (in mg/m 2) in these species. These data support the expectation that [ 18F]FPS will be safe for use in human PET imaging studies at a maximum administration of 5 mCi and a mass dose equal to or less than 2.8 μg FPS per injection.

Modulation of dopamine uptake in rat nucleus accumbens: effect of specific dopamine receptor antagonists and sigma ligands

The ability of dopamine (DA) antagonists and sigma receptor ligands to alter [ 3H]-DA uptake was examined using synaptosomes prepared from the nucleus accumbens of female rats. Pre-incubation with compounds having a high affinity for sigma (rimcazole, haloperidol, and spiperone) receptors produced dose dependent inhibition of 3H-DA uptake. Sulpiride, a pure DA D 2 antagonist had no effect. In contrast, DA uptake was potentiated in response to (+)-3-(3-hydroxyphenyl)-N-(1-propyl)piperidine, a mixed sigma receptor antagonist and DA D 2 receptor agonist. Similarly, SKF-10,047, a selective sigma receptor agonist, and progesterone, a putative endogenous ligand for the sigma receptor, produced significant increases in 3[H]-DA uptake. These data suggest a potential role for sigma and DA ligands in the regulation of DA uptake in the nucleus accumbens.

ChemInform Abstract: (2R-trans)-2-Butyl-5-heptylpyrrolidine as a Potent Sigma Receptor Ligand Produced by Streptomyces longispororuber.

A potent sigma (sigma) receptor ligand was isolated from the culture broth of Streptomyces longispororuber #525. The active compound was identified to be (2R-trans)-2-butyl-5-heptylpyrrolidine by spectroscopic and chemical studies. The compound exhibited high affinity and selectivity for sigma receptors. The IC50 values toward sigma1, sigma2 and dopamine D2 receptors were 2.0, 22.7 and more than 40,000 nM, respectively. Its (2S-trans)- and (+/-)-cis-isomers, both synthesized, were also found to be high affinity sigma ligands.

(2R-trans)-2-butyl-5-heptylpyrrolidine as a potent sigma receptor ligand produced by Streptomyces longispororuber.

A potent sigma (sigma) receptor ligand was isolated from the culture broth of Streptomyces longispororuber #525. The active compound was identified to be (2R-trans)-2-butyl-5-heptylpyrrolidine by spectroscopic and chemical studies. The compound exhibited high affinity and selectivity for sigma receptors. The IC50 values toward sigma1, sigma2 and dopamine D2 receptors were 2.0, 22.7 and more than 40,000 nM, respectively. Its (2S-trans)- and (+/-)-cis-isomers, both synthesized, were also found to be high affinity sigma ligands.

Occupancy of striatal D(2)-like dopamine receptors after treatment with the sigma ligand EMD 57445, a putative atypical antipsychotic.

EMD 57445 (panamesine) is a high affinity sigma ligand with the profile of an atypical antipsychotic in animal studies. It has been reported recently to have antipsychotic activity in schizophrenia. However, its metabolite, EMD 59983, binds also to D(2) and D(3) dopamine (DA) receptors. The aim of this study was to test, using single photon emission computed tomography (SPECT) and [(123)I]iodobenzamide (IBZM) as the radiotracer, whether EMD 59983 would pass the blood-brain barrier and to what extent it would contribute to the effects of EMD 57445 in schizophrenia. Two IBZM SPECT-scans were performed in five neuroleptic-free schizophrenic patients (DSM IV), one before and one after treatment with 60 mg panamesine daily for a treatment duration of 12-26 days. A high occupancy of striatal D(2)-like DA receptors similar to that induced by typical neuroleptics was observed in all patients treated with EMD 57445. Our results suggest that a possible antipsychotic activity of EMD 57445 in schizophrenia is not necessarily attributable to its affinity for sigma receptors, but could be simply due to the potent antidopaminergic effects of EMD 59983, its main metabolite.

Sigma Receptor Photolabeling and Sigma Receptor-mediated Modulation of Potassium Channels in Tumor Cells

Recent work has indicated that sigma receptor ligands can modulate potassium channels. However, the only sigma receptor characterized at the molecular level has a novel structure unlike any other receptor known to modulate ion channels. This 26-kDa protein has a hydropathy profile suggestive of a single membrane-spanning domain, with no apparent regions capable of G-protein activation or protein phosphorylation. In the present study patch clamp techniques and photoaffinity labeling were used in DMS-114 cells (a tumor cell line known to express sigma receptors) to investigate the role of the 26-kDa protein in ion channel modulation and probe the mechanism of signal transduction. The sigma receptor ligands N-allylnormetazocine (SKF10047), ditolylguanidine, and (+/-)-2-(N-phenylethyl-N-propyl)-amino-5-hydroxytetralin all inhibited voltage-activated potassium current (IK). Iodoazidococaine (IAC), a high affinity sigma receptor photoprobe, produced a similar inhibition in IK, and when cell homogenates were illuminated in the presence of IAC, a protein with a molecular mass of 26 kDa was covalently labeled. Photolabeling of this protein by IAC was inhibited by SKF10047 with half-maximal effect at 7 microM. SKF10047 also inhibited IK with a similar EC50 (14 microM). Thus, physiological responses to sigma receptor ligands are mediated by a protein with the same molecular weight as the cloned sigma receptor. This indicates that ion channel modulation is indeed mediated by this novel protein. Physiological responses were the same when cells were perfused internally with either guanosine 5'-O-(2-thiodiphosphate) or GTP, indicating that signal transduction is independent of G-proteins. These results demonstrate that ion channels can be modulated by a receptor that does not have seven membrane-spanning domains and does not employ G-proteins. Sigma receptors thus modulate ion channels by a novel transduction mechanism.

The anticonvulsant actions of sigma receptor ligands in the Mg2+-free model of epileptiform activity in rat hippocampal slices.

1. The anticonvulsant potency of a series of structurally-dissimilar compounds which possess nanomolar affinities for high-affinity sigma binding sites was examined in the Mg2+-free model of epileptiform activity in rat hippocampal slices. Extracellular field potential recordings in the CA1 region were employed to examine the effects of test compounds on spontaneous epileptiform activity and multiple population spikes evoked by stimulation of the Schaffer collateral-commissural pathway. 2. Applied at sigma site-selective (i.e. nanomolar) concentrations, dextromethorphan, ditolylguanidine, caramiphen and opipramol failed to modify Mg2+-free epileptiform activity; neither pro- nor anticonvulsant effects were observed. However, applied at micromolar concentrations, these and additional test compounds reversibly inhibited orthodromically-evoked epileptiform field potentials with a rank order potency (IC50 values in microM): dextrorphan (1.5) > ifenprodil (6.3) > dextromethorphan (10) > ditolylguanidine (15) > loperamide (28) > carbetapentane (38) > caramiphen (46) > opipramol (52). Micromolar concentrations of the same compounds also inhibited spontaneous epileptiform bursts recorded during perfusion with Mg2+-free medium. 3. Co-application of ropizine (10 microM), an allosteric modulator of dextromethorphan binding to high-affinity sigma receptors, failed to endow dextromethorphan 10 nM with anticonvulsant properties and did not modify the anticonvulsant potency of 10 microM dextromethorphan. 4. The effects of dextrorphan (10 microM), ifenprodil (20 microM), loperamide (50 microM) and caramiphen (100 microM) were examined in the presence of external Mg2+ on field potential input/output (I/O) relationships and paired-pulse facilitation (PPF) of field excitatory postsynaptic potentials. Only caramiphen elicited effects on these parameters, affecting synaptic transmission at the point of synaptic transfer and depressing PPF ratios to below baseline values. The effects of caramiphen on I/O relationships mimicked those of the established anticonvulsant adenosine: in contrast, adenosine evoked an increase in PPF ratios. 5. Because anticonvulsant activity was observed only at micromolar concentrations of the sigma ligands tested, the results indicate that their anticonvulsant actions should not be ascribed to their occupancy, observed at nanomolar concentrations, of high-affinity sigma binding sites. Rather, anticonvulsant activity more likely reflects functional NMDA receptor antagonism and/or blockade of high voltage-activated Ca2+ channels, effects which are associated with micromolar concentrations of the test compounds. Modulation of GABAergic inhibitory mechanisms may also contribute to the anticonvulsant properties of caramiphen.

Microinjection of sigma ligands into cranial nerve nuclei produces vacuous chewing in rats

Many typical neuroleptics carry a high risk for producing motor side effects in humans, and have significant affinities for sigma (sigma) receptors. Sigma receptors are densely concentrated in cranial nerve nuclei that comprise the final common pathways for lingual, facial and masticatory movements; thus, they may serve as important substrates for some of the unwanted movements that can accompany neuroleptic treatment. Therefore, the purpose of this study was to evaluate whether microinjection of sigma ligands into the facial nucleus or spinal trigeminal nucleus, oralis would cause orofacial dyskinesias, and whether these effects could be attenuated with sigma receptor antagonists. Microinjection of the high affinity sigma ligands, di-o-tolylguanidine or haloperidol (0-10 nmol/0.5 microl), produced a marked increase in vacuous chewing and facial tremors in rats, while coadministration of the functional sigma antagonists, BD1047 or BD1063 (5 nmol), greatly attenuated these drug-induced movements. Sulpiride and clozapine (10 nmol/0.5 microl), sigma inactive/dopamine active atypical antipsychotic drugs with a much reduced risk for producing motor side effects in humans, were unable to elicit orofacial dyskinesias when microinjected into the facial or spinal trigeminal nucleus, oralis. These studies indicate that sigma receptors may contribute to some forms of motor side effects resulting from antipsychotic drug treatment.